The invention relates to a display device having a first substrate comprising channels which contain an ionizable gas, in which a wall of a channel is provided with at least one electrode for generating, in operation, a plasma discharge of the ionizable gas, a second substrate provided with column electrodes, and a layer of electro-optical material between the two substrates.
Display devices for displaying monochromatic or color images comprise, inter alia, plasma-addressed liquid crystal display devices referred to as PALC displays. The PALC displays are used as television and computer monitor displays and are preferably of the flat-panel type.
A display device of the type described in the opening paragraph is known from PCT patent application WO-A-99/04408 (PHN 16.485). The flat-panel display device described in this application comprises a display screen having a pattern of (identical) data storage or display elements and a multitude of channels. The channels are filled with an ionizable gas and provided with electrodes for (selectively) ionizing the ionizable gas in operation. In the known display device, the channels have the shape of parallel, elongate channels (formed in a channel plate), which function as selection means for the display device (the plasma-addressed row electrodes). By applying a DC voltage difference across the electrodes in one of the channels of the channel plate, electrons are emitted (from the cathode) and ionize the ionizable gas, forming a plasma (plasma discharge). When the voltage across the electrodes in the one channel is switched off, and the gas is de-ionized, a subsequent channel is switched on. The channels are sealed by a (thin) dielectric layer (xe2x80x9cmicrosheetxe2x80x9d) on the display screen side of the display device. The display device further comprises a layer of an electro-optical material provided between the channel plate and a substrate provided with further electrodes which function as the data electrodes or column electrodes of the display device. The display device is formed by the assembly of the channel plate with the electrodes and the ionizable gas, the dielectric layer, the layer of the electro-optical material and the substrate with the further electrodes. The display elements are defined by the overlapping portions of the column electrodes and the channels of the channel plate.
Usually, a liquid crystal material is used as an electro-optical medium, in which the direction of polarization of incoming light changes in dependence upon the voltage across the liquid crystal layer. The light source is placed behind the channel plate. In a transmissive display, polarized light then passes the channel plate. This channel plate may be manufactured in different ways.
A first possibility is to start from an optically planar glass on which rib structures are made by means of powder blasting in a frit provided on the glass or by means of silk screen printing. Both technologies are costly, while, moreover, the upper sides of the ribs must be polished at a later stage so as to obtain a uniform thickness as well as prevent depolarization. Depolarization occurs also on the walls of the channels.
A second possibility is to provide the channels by means of powder blasting or by sawing in a glass plate. An optically planar and straight bottom is difficult to obtain with this method, so that depolarization also occurs in this case.
The rate at which the device can be driven, both as regards transmissive and as regards reflective display devices, is also dependent on the size (the width) of the channels because this influences the switch-off (extinction) of the plasma in the channels and notably defines the period of time in which plasma particles can reach the walls where they are to be de-ionized or neutralized. It is true that this period of time can be decreased by making use of, for example, a helium-hydrogen gas mixture instead of the conventional helium or helium-xenon gas mixtures, but this solution requires a hydrogen buffer.
Also the use of a microsheet presents problems because strict requirements are to be imposed on the (mechanical) flatness in this case. Moreover, the greater part of the applied voltage will be present across this microsheet so that high drive voltages, in this case column voltages, are necessary. Possible corrections of column voltages for compensating crosstalk or temperature variations will then increase accordingly, which usually leads to unacceptably high column voltages.
It is, inter alia, an object of the present invention to obviate one or more of the above-mentioned drawbacks as much as possible. To this end, a display device according to the invention is characterized in that the first substrate is provided at the area of crossings of the channels and the column electrodes with picture electrodes whose surface extends in the direction of the column electrodes through at least twice the width of a channel. The surface preferably extends in the direction of the column electrodes through at least 4 times the width of a channel.
Since the channels are now much narrower than the dimensions of the picture electrodes, the polarized light is not depolarized or hardly depolarized at the location of the picture electrodes. Locations where possible depolarization may occur, notably the (walls of the) channels may be covered with a black mask, if necessary.
Besides, the dimensions of the channels may be considerably smaller so that the plasma extinguishes at a faster rate. Instead of a helium-hydrogen gas mixture, use can now be made of helium or of the conventional helium or helium-xenon gas mixtures. This applies to both transmissive and reflective display devices.
A first embodiment of a display device according to the invention is characterized in that the picture electrodes are present on the side of a transparent dielectric layer remote from the layer of electro-optical material and extend across the channels. The picture electrodes are now realized as separate image faces on the dielectric layer (xe2x80x9cmicrosheetxe2x80x9d). This provides the possibility of compensating column crosstalk because the potential on the picture electrodes is unambiguous.
A preferred embodiment of a display device according to the invention is characterized in that, viewed in a cross-section, at least a part of the first substrate substantially completely surrounds the channels, and the picture electrodes are connected in an electrically conducting manner to at least a part of the walls of the channels via electrically conducting connections.
In this case, the dielectric layer (xe2x80x9cmicrosheetxe2x80x9d) is completely superfluous so that a more robust construction is obtained which is even cheaper. The channels may be manufactured by means of powder blasting, sawing or etching in glass, which is cheaper than powder blasting in frit or silk screening. Moreover, the voltages used are much lower than in the conventional PALC display devices. Due to these lower voltages, cheaper drive electronics are possible because the column crosstalk can also be compensated in a simpler way.
A further display device according to the invention is characterized in that the first substrate comprises a first sub-substrate in which parts of the walls of the channels are present on the side remote from the layer of electro-optical material, and a second sub-substrate provided with at least one electrode for generating, in operation, a plasma discharge of the ionizable gas.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.